The prior art provides methods for manufacturing expandable thermoplastic polymeric materials, including both crystalline and amorphous polymers. Production of expandable thermoplastic materials made from crystalline polymers is effected by heat plastifying a normally-solid polymer resin, admixing such heat plastified resin with a volatile blowing agent under heat and pressure to form a flowable gel, and thereafter extruding the gel into a zone of lower pressure and temperature to expand and cool the gel to form the desired solid polymeric foam product. 1,2-dichlorotetrafluoroethane has been widely used as such a blowing agent owing to the sufficient degree of dimensional stability it confers to masses of foamed products during the curing period. However, the obtention of success with other blowing agents such as hydrocarbons including alkanes such as pentane has recently been realized owing to the use of stability control agents such as those long-chain fatty acid/polyol partial esters described in U.S. Pat. No. 3,644,230 and the esters of high fatty acids detailed in U.S. Pat. Nos. 4,214,054 and 4,395,510, the entire contents of all of which are herein incorporated by reference. Therefore, the production of a wide variety of expandable thermoplastics comprising even crystalline polymers is now possible using conventional techniques. In known processes, a blowing agent is compounded into the starting polymer resin blend in proportions to make the desired degree of expansion in the resulting foamed cellular product, usually up to about a 60-fold volume expansion to produce products having aged foam densities down to about 9.6 kg/m.sup.3 (about 0.6 pounds per cubic foot). The expandable compositions may be prepared by heat plastifying an olefin polymer resin and mixing the resin with a stability control agent and a blowing agent, and finally activating the blowing agent to expand the mixture. Typical processes for producing such materials are outlined in U.S. Pat. Nos. 4,694,027; 4,894,395; 5,304,580; and 5,605,937, the entire contents of which are herein incorporated by reference thereto. However, the known means for producing expandable thermoplastic materials produce particles having less than optimal properties. An example of this is that expandable polyolefin particles such as polyethylene must be pre-expanded prior to impregnation with blowing agent, and such pre-expansion requirement is well-known to those skilled in this art. This necessary pre-expansion equates with a proportional increase the warehousing space required to store the finished expandable particles. Additionally, the shelf life of pre-expanded thermoplastic expandable materials made from normally-crystalline polymers is well-known by those skilled in this art to be far less than desirable.
Production of expandable compositions comprising amorphous polymers, on the other hand, such as polystyrene materials, include a commonly-used batch process wherein raw materials including an aqueous suspension containing styrene monomer, a surfactant, catalyst, and other additives are processed by well-known means under conditions of vigorous agitation to yield spherical polystyrene beads having diameters in the range of about 0.3 to 1.5 millimeters. Such beads are subsequently impregnated with a hydrocarbon or other blowing agent, typically pentane or halogenated hydrocarbons, by charging approximately one hundred parts (by weight) of polymer particles, one hundred parts water, and one part of a concentrated surfactant (or a mixture thereof, such as a mixture of an arylalkyl polyether alcohol and dodecyl benzene sulfonate) into an autoclave along with about three to ten parts of pentane. The mixture is heated to about 170 degrees Fahrenheit under pressure for 3 hours and cooled to room temperature to yield expandable polystyrene beads which are subsequently rinsed, dried, and stored for later use. Generally, in such a batch system, the size of the beads can be controlled by advantageously selecting the processing conditions, including: time, agitation, temperature, pressure, and reactant concentration. However, this process for producing expandable polystyrene beads is not without several inherent disadvantages, one being that large volumes of water are required to suspend the polystyrene beads during both the polymerization and the impregnation steps. This in reality is a two-fold disadvantage. First of all, the heat energy input to the water to bring it to the required temperature is forfeit at the end of the impregnation when the water is discharged to the environment. Secondly, prior to such discharge the water must be treated, at additional expense, in order to meet the strict regulatory requirements imposed by various governmental entities. These combined energy and treatment costs are a significant portion of the cost of expandable polystyrene which is passed on to the end-users of the polystyrene.
Another disadvantage is that the stirred tank reactors in which the suspension is reacted are vulnerable to a multiplicity of events which can cause the reactor agitator to cease in its motion, including power and equipment failures. Such failures represent potential catastrophic occurrences in that coalescence of monomer droplets may lead to formation of a single, large, reactive mass of material undergoing an uncontrollable, strongly exothermic reaction owing to the lack of effective means for heat removal in such instances. In addition to fouling of equipment, such a scenario represents a significant safety hazard.
Expandable thermoplastic polymeric materials, including both those prepared from both crystalline and amorphous materials in accordance with the foregoing containing volatile agents are known to be expandable by application of heat under conditions which permit the volatile agent to be vaporized and form large numbers of cells within the bulk of the polymeric material. (U.S. Pat. Nos. 4,174,427; 5,000,891; 5,240,657; 5,525,637; and 5,573,790, the contents of which are herein incorporated by reference, are descriptive thereof.) When heating of expandable thermoplastic materials is performed within the confines of a suitable mold, various articles of manufacture may be produced using the expandable polymeric materials mentioned, regardless of whether the materials exist in the form of pellets or beads. It is towards alternative means for preparing expandable crystalline and amorphous particles, including polyolefin and polystyrene beads, in avoidance of the disadvantages associated with the prior art processes and properties described above which the instant invention is concerned. The instant invention also relates to a means for producing expandable polymeric particles of reasonable uniformity in size, to reduce and even preclude the necessity for multi-step particle size classification of the spheroids. The spheroid particles produced in the present invention may contain various additives which include but are not limited to: flame retardants, nucleating agents, and other chemical species which are known to impart desirable properties to polymers. Such other chemical species are well-known to those of ordinary skill in the art.